| Research on nanotechnology has become increasingly popular because of their unique physical, chemical, optical and catalytic properties compared to their bulk counterparts. Among II-VI semiconductor materials, Zn O is a exceptional and eminent n-type semiconductor material with wide band-gap(3.37 e V) and the large exciton binding energy(60 me V) at room temperature. Owing peculiar characteristics such as high thermal conductivity, high electron mobility, good transparency, and simplicity to fabricate diverse nanostructures, Zn O nanostructures have drawn great attention among the researchers. Aimed to gas-sensing applications, it has fascinated a great consideration due to their advantageous features, for example high sensitivity to numerous oxidizing and reducing gases, in expensive, ease in sensor fabrication, and reduced size as well. Particularly, Zn O shows potential material platforms for the applications as a gas sensor rest with twinned, multilayered and stacked structures with a dimension of micro and nanostructures build up by low dimensional nano-building blocks as probable. On the other hand, secondary growth Zn O along with mirror symmetry nanostructures has been suspected to enable modulation of surface charge states of Zn O, modifying significantly its functionality. In this dissertation, we account a facile and tunable synthesis of Zn O with a variety of hexagonal, twinned and multilayered disk like nanostructures via simple hydrothermal process. The surface morphology, structural, optical and electrical properties of the synthesized Zn O nanostructures are characterized using XRD, SEM, HRTEM, gas sensing and, UV Visspectroscopy. The formation of nanostructures was attributed due to effect of surfactant concentration, working temperature, role of capping agent, and the charge transformation during the reaction system. In addition, the crystal structure, morphologies, growth mechanism are convoluted, and their sensing properties on the sensitivity, response-recovery time, stability, and selectivity are also examined. The main contents discussed, are summarized as follows:Hexagonal Zn O nanodisk structures were successfully prepared by using two surfactants via a simple controlled hydrothermal method. It was found that sodium dodecyl sulfate(SDS) and cetyltrimethylammonium bromide(CTAB), when added together in the same ratios, they exhibit an anisotropic growth in the lateral direction due to replacement of nitride counter-ions with hydroxide ions. Due to high surface area, these nanostructures showed excellent high gas sensing performances. The gas-sensors have been fabricated based on the disk nanostructures and tested towards reductive gases. The responses towards formaldehyde were 81.6 and 43.2, response and recovery times of(5, 7s) and(9, 11s) with an optimal temperature of 300 oC and 150 ppm respectively.On the basis of hexagonal Zn O nanodisk structures, mirror symmetric hexagonal nanodisks are successfully fabricated. The twinning of nanostructure is achieved by the synthesis of individual anionic and cationic surfactants. In the final solution, both the surfactants were used together to obtain twinning of nanodisks at calcinations time 12 and 24 h were approximately 200 and 750 nm in thicknesses, respectively. The addition of sodium sulfate served as electrostatic binder for mirror symmetric and the counter ions of NO32- with bromide ions produced basic micelles for Zn O disk-like nanostructures.We have successfully prepared the nanorod-embedded Zn O a nanodisk with average length of nanorods is 300-700 nm, using a low temperature hydrothermal approach. The secondary embedded Zn O nanorods were successfully grown on the surface of Zn O microdisks along the [0001] plane. For the embedding of nanorods, Zn O nanodisks were used as fundamental nanostructures synthesized by sodium dodecyl sulfate solution. The urea is used as a secondary precursor and it caused secondary nucleation at the [0001] polar surfaces of nanodisks. The gas-sensing properties for secondary nanostructure are also explored for different gases at various concentrations. The maximum responses at 200 °C were 24.7, 17.8, 8.5, 7.4, and 6.2 for formaldehyde, ethanol, methanol, acetone, and ammonia, respectively. The fabricated sensor showed a high selectivity and response toward formaldehyde prominently than toward other gases. This trend might be attributed to the dissociation energy. The selectivity and response of embedded Zn O nanorods gas sensor to formaldehyde was superior to those for other gases. The optimal temperature of 200 °C found suitable for the detection of 150 ppm concentrations, where the response was highest for all gases.We have successfully reported the preparation of Zn O hexagonal nanocones,with the assistance of double anionic surfactants. The XRD exhibited wurtzite Zn O nanostructures whereas FESEM showed an average length and base-diameter of 1.3 and1.2 μm. The HRTEM results showed that growth appeared mainly along(0001) andpolar surfaces. The damping of nanostructures turned into nanocones because of the anionic properties of PVP. The as prepared Zn O nanocones are exposed to formaldehyde for different concentration(50-350) ppm at temperature ranges of(100-400) °C. It was found that at concentration 350 ppm and 275 °C, the sensor showed a response 12.5, with a sharp response and recovery time(8, 9 s) respectively.We have efficiently fabricated Multi-layered Zn O circular nanodisks using sodium dodecyl sulfate(SDS) and polyvinylpyrrolidone(PVP) polymer by controlled hydrothermal method. The addition of SDS provided electrostatic binding for nano mirrors-structures while PVP reproduced the reverse micelle microemulsion in the multilayered circular nanosdiks. The gas-sensing properties of multi-layered nanodisks are also reported for formaldehyde under different concentrations from 5 to 1200 ppm at 300 °C. Gas sensitivities measured at 5, 15, 30, 60 and 100 ppm were 15.2, 28.5, 39.8, 70.6 and 82.7 respectively. The response and recovery time of the sensors were 5 and 7s respectively. A higher response toward formaldehyde was observed, rendering the multi-layered nanostructures a promising gas-sensing material for the on-site detection of formaldehyde.Morphological transformation is successfully achieved from Zn O hexagonal needle-like rods to hexagonal flower-like rods by varying the reaction growth time using simple hydrothermal technique. The FESEM results show that average length and diameter of needle-like rods is 3.4 to3.8 μm and 350 to 400 nm, respectively. The average length and diameter of flower-like nanostructure is 100 to 100 nm and 700 t 850 nm, respectively. We find that HMT served as an OH- ions liberating agent and MEA has vital role in the aggregation of needle-like nanorods to make nanoflowers. The Optical bandgap energies are 3.317 and 3.323 e V obtained from the absorption spectra using UV-vis spectroscopy. The gas-sensing measurements reveal that the sensor made of nanoflowers shows sensitivity as high as 45.7 to ethanol gas and 33.6 to NO2 gas at concentration of 50 ppm and optimal temperature of 350 °C. The fabricated sensor displayed lower response and recovery times for NO2(6 and 12 s) than those for C2H5OH(10 and 7 s) at 350 °C and 50 ppm. |
PDF Full Text Request